There are many devices for detecting radiation. In one type of detector, photocathodes are used with microchannel plates (MCPs) to detect low levels of electromagnetic radiation. Photocathodes emit electrons in response to exposure to photons. The electrons can then be accelerated by electrostatic fields toward a microchannel plate. The microchannel plate produces cascades of secondary electrons in response to incident electrons. A receiving device then receives the secondary electrons and sends out a signal responsive to the electrons. Since the number of electrons emitted from the microchannel plate is much larger than the number of incident electrons, the signal produced by the device is amplified for viewing by an observer.
One example of the use of a photocathode with a microchannel plate is in an image intensification device. The image intensification device is used in night vision devices to amplify low light levels so that a user may see even in very dark conditions. In the image intensification device, a photocathode produces electrons in response to photons from an image. The electrons are then accelerated to the microchannel plate, which produces secondary emission electrons in response. The secondary emission electrons are received at a phosphor screen or, alternatively, a charge coupled device (CCD), thus producing a representation of the original image.
Image intensification devices are constructed for a variety of applications, and, therefore, vary in both shape and size. These devices are particularly useful for both industrial and military applications. For example, image intensification devices are used in night vision goggles for enhancing the night vision of aviators and other military personnel performing covert operations. They are also employed in security cameras, photographing astronomical bodies and in medical instruments to help alleviate conditions such as retinitis pigmentosis, more commonly known as night blindness. Such an image intensifier device is exemplified by U.S. Pat. No. 5,084,780, entitled TELESCOPIC SIGHT FOR DAY/NIGHT VIEWING by Earl N. Phillips, issued on Jan. 28, 1992, and assigned to ITT Corporation, the assignee herein.
Image intensification devices are currently manufactured in two types, commonly referred to as Generation II (GEN 2) and Generation III (GEN 3) type image intensifier tubes. The primary difference between these two types of image intensifier tubes is in the type of photocathode employed in each. Image intensifier tubes of the GEN 2 type have a multi-alkali photocathode with a spectral sensitivity in the range of 400–900 nanometers (nm). This spectral range can be extended to the blue or red by modification of the multi-alkali composition and/or thickness. GEN 3 image intensifier tubes have a p-doped gallium arsenide (GaAs) photocathode that has been activated to negative electron affinity (NEA) by the absorption of cesium and oxygen on the surface. This material has approximately twice the quantum efficiency (QE) of the GEN2 photocathode. An extension of the spectral response to the near infrared can be accomplished by alloying indium with gallium arsenide.
A transmission type of photocathode refers to a photocathode in which light energy strikes a first surface and electrons are emitted from an opposite surface. Photocathodes as used in modern night vision systems operate in a transmission mode.
A conventional method of fabricating a negative electron affinity transmission device involves the synthesis of a single photosensitive material that is deposited or bonded onto a transparent substrate. Fabricating a photocathode for a GEN2 image intensification device involves the deposition of a bi-alkali material onto a glass substrate, or faceplate. The faceplate's optical properties are such that it is predominately transparent to light of wavelengths that are absorbed by the photosensitive material.
A similar method is used to fabricate a GEN3 photocathode by using a photosensitive single crystal semiconductor material, such as Gallium Arsenide (GaAs). The thin GaAs film is typically thermally bonded to the transparent faceplate, by methods known to those skilled in the art of making image intensifiers.
During operation of the image intensification device, a photon that passes through the faceplate may be absorbed by the photosensitive material and create an excited electron within the material with an energy transition equal to the absorbed photon energy. This electron may then diffuse to the photosensitive material/vacuum interface and be emitted into a vacuum with a finite probability. In the case of GEN3 GaAs photocathodes, photons that are transmitted through the faceplate glass with energy greater than the fundamental band gap energy of GaAs, may be absorbed and create excited electrons.
The bandwidth, or spectral photosensitivity range, for an ideal GEN3 GaAs photocathode spans the energy range from the transmission edge of the glass faceplate to the fundamental band gap energy of GaAs. For typical faceplate glass formulations, the high energy transmission edge is approximately 350 nm. The fundamental band gap energy for GaAs is 880 nm. An ideal spectral photosensitivity in terms of quantum efficiency (QE) may have the characteristics shown in FIG. 5.
In practice, however, defects in the GaAs material and at the GaAs/glass interface decrease the diffusion lifetime of photo excited electrons. This may drastically reduce the photo sensitivity (photo response), especially at the short wavelength region of FIG. 5. Reduction of defects near the GaAs/glass interface may be accomplished by monolithically depositing a lattice matched layer onto the GaAs absorption layer, which is transparent to the wavelengths of interest.
A lattice matched layer, commonly used, is a semiconductor material alloy AlxGa1-xAs, also called a window layer. Using deposition techniques, high quality AlGaAs/GaAs interfaces may be produced that result in reduction of interface defects by several orders of magnitude. A known method is to deposit a window layer that has high optical transmission properties in the 350–900 nm range to achieve a broad spectral response. Typical GEN3 GaAs transmission photocathodes achieve a spectral response bandwidth of 500–900 nm, using an Al0.8Ga0.2As alloy for the window layer composition.
An anti-reflective coating (ARC), such as Si3N4 may also be added at the glass/AlGaAs interface. This then results in layers of glass/Si3N4/Al0.8Ga0.2As/GaAs, which represent a conventional GEN3 transmission photocathode. The goal for this GEN3 photocathode, as well as a typical alkali metal GEN2 photocathode, is to maximize their spectral bandwidth photo-response.
A GEN 3 image intensifier tube according to the prior art is illustrated in FIG. 6. Image intensifier tube 10 includes an evacuated envelope or vacuum housing 22 having photocathode 12 disposed at one end of housing 22 and a phosphor-coated anode screen 30 disposed at the other end of housing 22. Microchannel plate 24 is positioned within vacuum housing 22 between photocathode 12 and phosphor screen 30. Photocathode 12 includes glass faceplate 14 coated on one side with an antireflection layer 16; an aluminum gallium arsenide (AlxGa1-xAs) window layer 17; a gallium arsenide active layer 18; and a negative electron affinity coating 20.
Microchannel plate 24 is located within vacuum housing 22 and is separated from photocathode 12 by gap 34. Microchannel plate 24 is generally made from a thin wafer of glass having an array of microscopic channel electron multipliers extending between input surfaces 26 and output surfaces 28. The wall of each channel is formed of a secondary emitting material. Phosphor screen 30 is located on fiber optic element 31 and is separated from output surface 28 of microchannel plate 24 by gap 36. Phosphor screen 30 generally includes aluminum overcoat 32 to stop light reflecting from phosphor screen 30 from reentering the photocathode through the negative electron affinity coating 20.
In operation, photons from an external source impinge upon photocathode 12 and are absorbed in the GaAs active layer 18, resulting in the generation of electron/hole pairs. The electrons generated by photocathode 12 are subsequently emitted into gap 34 of vacuum housing 22 from the negative electron affinity coating 20 on the GaAs active layer 18. The electrons emitted by photocathode 12 are accelerated toward input surface 26 of microchannel plate 24 by applying a potential across input surface 26 of microchannel plate 24 and photocathode 12.
When an electron enters one of the channels of microchannel plate 24 at input surface 26, a cascade of secondary electrons is produced from the channel wall by secondary emission. The cascade of secondary electrons are emitted from the channel at output surface 28 of microchannel plate 24 and are accelerated across gap 36 toward phosphor screen 30 to produce an intensified image. Each microscopic channel functions as a secondary emission electron multiplier having an electron gain of approximately several hundred. The electron gain is primarily controlled by applying a potential difference across the input and output surfaces of microchannel plate 24.
Electrons exiting the microchannel plate 24 are accelerated across gap 36 toward phosphor screen 30 by the potential difference applied between output surface 28 of microchannel plate 24 and phosphor screen 30. As the exiting electrons impinge upon phosphor screen 30, many photons are produced per electron. The photons create an intensified output image on the output surface of the optical inverter or fiber optics element 31.